ZnO-based crystal such as ZnO:Ga, is a multifunctional semiconductor material with wide and direct band gap, combines the characteristics of transparent conduction, ultra-fast scintillation and ultraviolet emission, etc.
The crystal growth techniques such as hydrothermal, flux and vapor deposition method, etc. , were used for preparing bulk ZnO crystals. Up to now, only hydrothermal method was reported for GZO single crystal growing. The main drawback of this method is that the doped Ga2O3 of grown crystal is too little, less than 0.1 wt %. Furthermore, the maximum size of grown crystals is typically 30.44 mm×24.84 mm×5.40 mm, which is not large enough to meet the demand of in-depth research. The growth speed limited by slow solvent diffusion and mass transfer across the boundary, was about 0.1 mm per day, and the growth cycle is always about 20 days. So there are many difficulties in realizing the commercial production of GZO crystals by this method.
It is noteworthy that ZnO, not like GaN or GaAs, does not react with air in higher temperature, so that growing GZO crystal by high temperature flux method in air has great developing potential. Up to now, although some kinds of flux were used for growing ZnO crystal and some progresses were made, but there were still several problems remain to be resolved. Firstly, grown crystals are small in size and non-uniform in composition. Secondly, the flux always leads to the undesired impurity and stress. It is really detrimental to the properties of electric material that requires a high level in controlling of impurity content and stoichiometric ratio. Last but not least, that ZnO is volatile, is the main challenge for flux method. In short, that to find a more suitable flux and to optimize the process condition, call for further research.
Up to now, the flux reported for growing pure ZnO crystal contains PbF2, P2O5+V2O5, V2O5+B2O3,MoO3 and V2O5+MoO3. However, there is no report concerning in particular GZO crystals growing by flux method. An article (Journal of Crystal Growth 237-239 (2002) 509-513) reported using top-seeded solution growth and travelling solvent floating zone techniques to grow pure ZnO crystal, and find a new oxide combination 9.3% (B2O3) +16.3% (MoO3) +6.7% (Nb2O5) as flux, and grew out pure ZnO and GZO crystals with eleven kinds of doping content by travelling solvent floating zone method. The maximum of actual doped Gallium oxide weight percent can reach 1.0 wt %, which is far above 0.053 wt % of that grown by hydrothermal method. The grown GZO crystals are brown green in color and transparent, have maximum size typically with 12 mm in diameter and 120 mm in length, are bigger than 10 mm×5 mm×2 mm of reported pure ZnO grown by flux method. In our experiments, the selected flux reduces the growth temperature to below 1300° C. in which ZnO or GZO starts to evaporate, and suppresses the characterization of strong polarity crystallization based on ZnO-based materials. Additionally, travelling solvent floating zone technique is beneficial especially to ZnO-based crystals growing. For example, that real-time monitoring on melting and crystallizing make the composition of flux be stable in the molten zone. The high temperature gradient near solid liquid interface significantly increases the crystallization driving force and Gallium oxide doping amount.